Smith Lecture: New Insights Into Trophic Ecology of Marine Animals from Compound Specific Nitrogen Isotope Analysis of Amino Acids
Ecosystem based management requires an understanding of the inter-connectedness of organisms within an ecosystem. Arguably, the most important of these connections are trophic interactions, or the trophic position that an organism occupies within a given food web. The nitrogen isotopic composition of a consumer’s tissue has been used to investigate trophic linkages within marine food webs. However, there are a number of assumptions that must be met in order to satisfactorily interpret bulk tissue d15N values in terms of trophic position, including knowing the isotopic composition of plants at the base of the food web and the expected enrichment in 15N at each trophic step. Compound specific nitrogen isotope analysis of amino acids can alleviate some of the challenges associated with the interpretation of bulk tissue d15N values. In samples of consumer tissues some amino acids, such as phenylalanine, appear to retain the isotopic composition of nitrogen sources at the base of the food web, whereas other amino acids, such as glutamic acid, are significantly enriched in 15N with each trophic transfer. As such, the isotopic composition of the base of the food web and trophic information can be obtained from the tissue of only a consumer without need for analyzing prey items or basal food web resources. The application and pitfalls of this technique to marine organisms including zooplankton, reef and pelagic fishes, sea turtles and rays will be discussed. I will also emphasize the role that compound specific isotope analysis of amino acids can play in verifying or providing critical information for ecosystem models, which are often used to help understand and predict changes in ecosystems due to environmental variability and fisheries.